Can't handle empty glyph name

I.e. a `glyphname = "";` in the .glyphs file.

```
Traceback (most recent call last):
File "c:\program files\python38\lib\runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "c:\program files\python38\lib\runpy.py", line 87, in _run_code
exec(code, run_globals)
File "C:\Program Files\Python38\Scripts\glyphs2ufo.exe\__main__.py", line 7, in
File "c:\program files\python38\lib\site-packages\glyphsLib\cli.py", line 228, in _glyphs2ufo_entry_point
return main(args)
File "c:\program files\python38\lib\site-packages\glyphsLib\cli.py", line 189, in main
return options.func(options)
File "c:\program files\python38\lib\site-packages\glyphsLib\cli.py", line 208, in glyphs2ufo
glyphsLib.build_masters(
File "c:\program files\python38\lib\site-packages\glyphsLib\__init__.py", line 139, in build_masters
source.font.save(ufo_path)
File "c:\program files\python38\lib\site-packages\ufoLib2\objects\font.py", line 546, in save
self.write(writer, saveAs=saveAs)
File "c:\program files\python38\lib\site-packages\ufoLib2\objects\font.py", line 481, in write
self.layers.write(writer, saveAs=saveAs)
File "c:\program files\python38\lib\site-packages\ufoLib2\objects\layerSet.py", line 311, in write
layer.write(glyphSet, saveAs=saveAs)
File "c:\program files\python38\lib\site-packages\ufoLib2\objects\layer.py", line 328, in write
glyphSet.writeGlyph(
File "c:\program files\python38\lib\site-packages\fontTools\ufoLib\glifLib.py", line 455, in writeGlyph
fileName = self.glyphNameToFileName(glyphName, self._existingFileNames.values())
File "c:\program files\python38\lib\site-packages\fontTools\ufoLib\glifLib.py", line 573, in glyphNameToFileName
return userNameToFileName(glyphName, existing=existingFileNames, suffix=".glif")
File "c:\program files\python38\lib\site-packages\fontTools\ufoLib\filenames.py", line 76, in userNameToFileName
if not prefix and userName[0] == ".":
IndexError: string index out of range
```

Reproduce the failure with a .glyphs file containing glyphname = "" and trace the conversion from glyphsLib/cli.py through glyphsLib/__init__.py to the UFO save operation shown in the traceback. Done means the conversion handles the empty glyph name without the reported IndexError.

Written by the indexing model from the issue text.